A disc brake is a type of brake wherein a pair of braking pad assemblies are urged on both friction surfaces formed on a rotary disc to restrain the rotation of the disc, and it is widely used for cars and the like.
The disc brake is at present not free from a serious problem which is concerned with a great likelihood of giving rise to a so-called dragging phenomenon. That is, the braking pad assemblies which are urged onto the disc surfaces are liable to further remain thereon in the urged state at a certain amount of force, even after the brake has been released of its operation. When this dragging happens to force the disc to be rotated under the continuous dragging torque, the resultant energy loss will amount to more than a negligible value. It is frequently said that the reduction of the dragging torque of this kind during the non-braking operation period is a key to the attainment of the fuel consumption economy desired in cars at the present stage.
The dragging phenomenon often shortens life of the braking pad assemblies through increase of wearing, and further causes overheating of the brake system. The overheating decreases in turn the coefficient of friction of the braking pad assemblies, consequently deteriorates the braking effect, and it sometimes leads to the vapor-lock, when the braking fluid may be boiled by chance with a result of rendering the brake system incapable or inefficient in performing its function.